Time-temperature indicator devices provide a way of indicating a cumulative time-temperature exposure. A device may be capable of indicating whether a commodity has been exposed to a temperature greater than a predetermined temperature for a period of time. For example, a device might indicate exposure to an excessive temperature for an excessive time or exposure to an adequate temperature for an adequate time.
Various types of time-temperature indicator devices have been disclosed which rely on the diffusion of a dye through a polymer to provide a visual indication of cumulative time-temperature exposure.
U.S. Pat. No. 4,212,153, issued Jul. 15, 1980, titled "Time Color Indicator," by Kydonieus et al., discloses a time indicator device in which molecular migration of a dye or other agent from one surface of a polymeric indicator layer to the other causes a visible change after a defined time. The indicators exemplified make use of plasticized polyvinyl chloride as the polymeric material. Examples are shown where the rate of appearance of dye colour is accelerated by higher temperatures.
U.S. Pat. No. 5,057,434, issued Oct. 15, 1991, titled "Multifunctional Time-Temperature Indicator," by Prusik et al., describes an integrated time-temperature indicator device comprising a system of printable multilayer compatible compositions including (i) a layer of a first composition (a) which develops a first color change as a result of a cumulative time-temperature exposure; (ii) a composition (b) that is capable of melting at a predetermined temperature; and (iii) a composition (c) capable of producing a second color change only as a result of the melting of composition (b) and wherein said second color change is combined with the first color change to yield an integrated color change result.
U.S. Pat. No. 5,476,792, issued Dec. 19, 1995, titled "Time-Temperature Indicator Devices," by Ezrielev et al., discloses a time-temperature indicator device for recording the duration of time over which a temperature has been established above a predetermined temperature. The device includes a dye-compatible polymer composition having a softening point above the predetermined temperature and a polymer-compatible dye composition in contact with the polymer composition and present in a quantity sufficient for diffusion through the polymer composition whenever it is above the predetermined temperature. The quantity of dye is selected so that the extent of diffusion corresponds to the duration of exposure above the predetermined temperature relative to the total time required for substantially complete diffusion. The devices exemplified are suitable for providing a heat history for medical instruments subjected to high temperature sterilisation.
There is a need for devices which provide an indication of cumulative time-temperature exposure which is not discernible by simple visual inspection. There is a need for devices which can be activated at the point of use without need for appropriate storage before use.